List comprehensions for Objective-C.
List comprehensions, as available in many other languages, are quite useful for building up lists out of other lists. Here is a simple example in Haskell:
[[a,b] | a <- [1,2,3], b <- [4,5,6]]
[[1,4],[1,5],[1,6],[2,4],[2,5],[2,6],[3,4],[3,5],[3,6]]
A comprehension takes a function and one or more lists, and returns a new list. Lists passed to the comprehension are enumerated from right to left in a nested fashion, that is, if you take the above example and translate it to vanilla Objecive-C it would look like this:
NSMutableArray array = [NSMutableArray array];
for(id obj in @[@1,@2,@3]) {
for(id obj1 in @[@4,@5,@6]) {
array addObject:@[obj, obj1]];
}
}
The plain Obj-C version is quite a bit more verbose and cumbersome. In comes TJLListComprehension.
NSArray *list1 = @[@1, @2, @3];
NSArray *list2 = @[@4, @5, @6];
NSArray *array = [TJLListComprehension listComprehensionWithLists:@[list1, list2] function:^id(id obj, id obj1) {
return @[obj, obj1];
}];
All three examples produce the exact same list, but the first and last example raise you up out of the muck of writing your own loops and mutable containers and lets you focus and what you actually want to accomplish. Also note that The function can return any object, but this example just uses making a new, multi dimensional list out of existing ones as an example.
It is also possible to filter on the lists with a predicate function:
NSArray *list1 = @[@1, @2, @3];
NSArray *list2 = @[@4, @5, @6];
NSArray *array = [TJLListComprehension listComprehensionWithLists:@[list1, list2] function:^id(id obj, id obj1) {
return @[obj, obj1];
} predicate:^BOOL(NSNumber *obj, NSNumber *obj1) {
return (obj.integerValue + obj1.integerValue) % 2 == 0;
}];
If the predicate returns YES, then the function will be applied and those elements added to the new list, if it return NO, then the function will not be applied and those elements not added to the list.
When creating a list comprehension with +TJLListComprehension listComprehensionWithLists:function:, you should pass a list of lists, as in the above example. Using an array literal is a convenient way to do this. Anything that conforms to NSFastEnumeration can be passed in for the lists parameter.
The function argument is typed as simply id, but it is a block that can take a variable number of arguments, which should be the same as the number of lists passed in the lists parameter.
TJLListComprehension uses cocoapods, so just put pod 'TJLListComprehension' 'version' into your podfile and install like normal. then just #import <TJLListComprehension/TJLListComprehension.h> wherever you want to use it.
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